I've heard of using Phosphoric Acid for dried grout that's been left behind on the SURFACE of the tile (from the grouting part of the job), but I'm not sure if it works on tile that has sanded or unsanded grout. (I'm NOT suggesting that you use the acid to 'soften' the grout that's BETWEEN the tile - Just the grout, if any, that's hardened on the surface of the tile. And it may not work!) As you will read, there are power tools for removing the grout BETWEEN tiles. You need to be very, VERY careful that your tool doesn't run amuck and damage the tile itself. The residual grout on the 'surface' of the tile (that's already dried), may not be able to be removed.